Power steering systems are used on many types of vehicles, including fork lift trucks and other industrial vehicles, for providing assistance to the operator in maneuvering the vehicle. The most common types of power steering systems are hydraulic, having a hydraulic pump which is driven by a V-belt from the engine or the accessory power section in a vehicled powered by an internal combustion engine, or by a separate electric motor in an electrically powered vehicle. The pump functions continously while the vehicle is being operated, thereby providing continuous hydraulic power to the steering system. If no steering commands are initiated by the operator of the vehicle, the fluid circulates freely within the system; however, if a steering command is given by the operator, the circulating hydraulic fluid is directed to one or the other side of a hydraulic steer cylinder. The cylinder is mechanically linked to a pitman arm or other linkage in the steering mechanism of the vehicle, and controls the position of the steer wheels of the vehicle.
The hydraulically operated power steering systems which are commonly used and have continuously operating hydraulic pumps, perform satisfactorily in providing power assisted steering; however, certain objectionable features are associated with these systems. The continuously operating hydraulic pump is energy wasteful, in that frequently no steering commands are given by the operator for relatively long periods of time, yet the pump operates continuously and circulates hydraulic fluid which is not required and serves no useful purpose at that time. The energy consumed in operating the pump is completely wasted when steering commands have not been given. In a vehicle powered by an internal combustion engine, whether of the gasoline, diesel, or propane type, as a result of the continuously operating pump, a greater amount of fuel is consumed than if the pump operated only when steering commands were given and pressurized fluid was needed. Hence, fewer miles of travel and operating hours per quantity of fuel are obtained because of the continuously operating pump. In electrically powered vehicles, the continuously operating hydraulic pump creates other undesirable effects. The energy loss from the electrical system to power the pump is dissipated in the form of heat, which can add to the discomfort of the operator, especially if the vehicle is being used in an already warm working environment. Additionally, it is desirable in electric vehicles to match all of the operating equipment, including the hydraulic power steering pump and motor, for the highest possible efficiency at the most economical size, thereby minimizing energy consumption. Normally, this would require a relatively small pump and motor operating at relatively high speeds. Such a combination has been found to be objectionable, however, in that the operation thereof results in higher noise levels than are desirable. Thus, pumps and motors which are larger than the optimal size have been used, since the larger pumps and motors can operate at slower and less noisy speeds. The larger pumps and motors are not only less efficient, thus resulting in higher energy consumption than small pumps and motors, but the larger pumps and motors are also more costly to obtain the install, hence adding to the overall vehicle cost.